Bearing Capacity Of Spudcan Foundation Research Articles

Effects of Prior Footprints on the Bearing Capacity of Spudcan Foundations: A Case Study

A jack-up rig is a self-elevating offshore drilling platform designed for mobile operations in the marine environment. The rig is supported by 3 or 4 legs, with spudcan foundations at the end. These spudcan provide the necessary stability and load-bearing capacity for rig foundation during drilling operations. The mobile rig sometimes revisits sites where prior installation, operation, and extraction activities have resulted in identifiable footprints on the seabed. The penetration of spudcan close to, or partially overlapping, footprints shall be carefully considered because the spudcan-footprint interaction may cause a change in the ultimate vertical bearing capacity (Fv). ISO 19905-1:2016 states that the interaction between a spudcan and a footprint is expected to be minimal when the edge-to-edge distance exceeds one spudcan diameter. This paper presents the effect of existing footprints on the vertical bearing capacity of a spudcan with various spacing variations from 0.5B to 1.75B, where B is the diameter of the spudcan. A 3D finite element method (3D FEM) is employed as the primary analysis tool in analyzing the effects of footprints on layered clay soil. The 3D FEM analysis is conducted because the analytical approach outlined in ISO 19905-1:2016 does not consider the influence of the footprint on the spudcan capacity. The analysis results show that bearing capacity may reduce by more than 50% when the center-to-center distance is less than 1B. Whereas, for distances greater than 1.5B, the influence of the footprint is relatively insignificant, only reducing less than 10% of the ultimate capacity of the spudcan.

Effects of spudcan installation processes on the bearing capacity of foundations under combined loading in soft-over-stiff clay

The bearing capacity of spudcan foundations under vertical, horizontal, and moment loading must be adequately assessed prior to the operation of jack-up platforms. However, studies the effects of combined loading on spudcan foundations in soft-over-stiff clays, considering the effect of installation processes, have been limited. In this study, the two-dimensional remeshing and interpolation with small strain method was used to simulate the large deformations experienced by the soil during the installation of a spudcan, and a three-dimensional model was employed for the small strain analysis regarding the bearing capacity. The undrained shear strength of stiff clay influenced the failure mechanisms of the soil during vertical and moment loading. However, it had minimal impact during horizontal loading. Therefore, in this study, an equation for the bearing capacity factors for vertical and moment loading conditions is proposed. The properties and parameters of the combined bearing capacity failure envelope with respect to different dimensions are analysed and corresponding equations for the failure envelope are proposed, which aid the on-site assessment of jack-up platforms prior to them becoming operational.

Probabilistic bearing capacity of spudcan foundations under combined loading in spatially variable soils

Spudcan foundations are widely used in the offshore gas and oil industry to support mobile jack-up drilling rigs. During the service period, the spudcan foundations need to resist large vertical-horizontal-moment (VHM) loads that are transmitted from the superstructure. The bearing capacity of foundations in complicated stress conditions is usually based on VHM failure envelopes. These are commonly constructed by using numerical methods where the soil properties are assumed to be deterministic, ignoring the inherent spatial variability. This paper investigates the influence of spatially variable undrained shear strength and embedment depth on the capacity of spudcan foundations under VHM combined loads. The linearly increasing undrained shear strength with depth obeying log-normal distribution was taken into account. Probabilistic failure envelops were constructed to analyse the effect of spatial variability on the bearing capacity of spudcan foundations at various embedment depths when subjected to different combinations of VHM loads. The influence of horizontal scale of fluctuation for a specific embedment depth was also investigated. The results indicated that under combined loading the embedment depth significantly influenced the shape and size of failure envelopes. The probability of failure at each embedment depth based on the obtained probabilistic failure envelopes was intuitionally represented by means of a factor of safety, which can provide specific guidance for foundation reliable designs.

Effects of installation on combined bearing capacity of a spudcan foundation in spatially variable clay

Mobile jack-up drilling rigs with spudcan foundations are extensively used in offshore industry due to their flexibility, mobility and cost-effectiveness. A sufficient bearing capacity of the spudcan foundation is needed to resist the combined environmental loading associated with storms, winds, waves and currents. In most previous studies, the combined bearing capacity was defined for the wished-in-place spudcan foundation, ignoring the disturbance to the seabed soils cause by spudcan installation. It is shown in this paper how the installation affects the failure mechanism and bearing capacity of spudcan foundation in spatially variable soil. Random large deformation finite element method is first used to simulate the installation process of a spudcan foundation in order to obtain the disturbed soil strength profiles. Then the profiles are mapped into random small-strain finite element model for calculation of bearing capacity of the spudcan foundation. Results show that a large amount of the softer soils in the surface of seabed flow back to top of spudcan foundation, which causes the bearing capacity to reduce significantly. Moreover, a Monte-Carlo simulation is carried out to define the probabilistic failure envelopes of the spudcan foundation, considering the disturbance of installation to the spatially variable soils.

Bearing capacity and reliability analysis of spudcan foundations embedded at various depths based on the non-stationary random finite element method

Spudcan foundations for mobile jack-up units are large diameter conical foundations that can penetrate into the seabed to a maximum depth of three times the diameter of the spudcan. As a deeply penetrated foundation, the bearing capacity of a spudcan is highly dependent on the failure mechanism and the strength distribution of the seabed soil, which exhibits strong spatial variability. While many recent studies have explored the failure mechanism and bearing capacity of spudcan foundations, these studies have commonly deterministically described the soil in which the spudcan is embedded. Therefore, this paper aimed to investigate the probabilistic bearing capacity of spudcan foundations embedded in the seabed at various depths, under the condition of spatially variable undrained shear strength that increased linearly with depth. The random finite element method was used to study this problem, where nonlinear finite element analysis was combined with random field theory within a Monte Carlo framework. The variability in undrained shear strength was modelled as a random field and characterised by a log-normal distribution with anisotropic autocorrelation distances. The influence of autocorrelation distance on the mean bearing capacity and the failure mechanism was discussed for each embedment depth. A correlation between the probability of failure and the factor of safety was established based on the obtained results. These findings can provide guidance for reliability-based design of spudcan foundations embedded in spatially variable soil.

Probabilistic bearing capacity analysis of spudcan foundation in soil with linearly increasing mean undrained shear strength

Spudcan foundations have been widely used in offshore oil and gas engineering for jack-up rigs. During penetration into seabed soil, spudcan foundations may encounter large uncertainties in soil properties, which have a strong impact on its bearing capacity. A large number of studies have been conducted to demonstrate the failure mechanisms and the bearing capacity of spudcan foundations in soils with deterministic properties, but the influence of spatially variable soil properties on spudcan foundations remains unclear and should be accounted for. In this study, we aimed to investigate the failure mechanisms and the bearing capacity of a spudcan foundation embedded in soils where the mean undrained shear strength increased linearly with depth. The spectral representation method (SRM) was used to generate a non-stationary non-Gaussian random field. The random finite element method (RFEM) in conjunction with Monte Carlo simulations was carried out to evaluate the statistical characteristics of the bearing capacity for spudcan foundations, followed by a series of comparisons and discussion. The results indicated that the mean bearing capacity for the spatial variability of undrained shear strength was smaller than that obtained from the corresponding deterministic case. The probability of failure decreased slightly with an increase in the degree of non-stationarity under the constant coefficient of variation (COV) condition. Upon comparing the results obtained from different random field models, we found that the probability of failure under the non-constant condition was smaller than that under the stationary and constant COV conditions.

Centrifuge Model Study on the Effect of Lattice Leg and Sleeve on the Postconsolidation Bearing Capacity of Spudcan Foundation

Up to now, the postconsolidation bearing capacity enhancement of jack-up spudcan foundation has been explored using centrifuge model tests and numerical analyses, which however ignored the realistic jack-up lattice leg. This paper investigates both typical lattice leg and sleeve effects on the postconsolidation spudcan bearing capacity using centrifuge model tests, by replicating the entire process of spudcan in normally consolidated clay: “penetration–unloading–consolidation–repenetration.” The experimental results show that the lattice leg and sleeve affect the spudcan bearing capacity in two sides compared with spudcan without leg. First, it increases the transient bearing capacity during initial spudcan penetration; second, less postconsolidation bearing capacity improvement is yielded by the presence of the leg. The former effect is of importance on the prediction of jack-up leg penetration, and the latter effect would suggest a lower risk of spudcan punch-through for realistic offshore jack-up rig during preloading and operation period.

Effect of Lattice Leg and Sleeve on the Transient Vertical Bearing Capacity of Deeply Penetrated Spudcans in Clay

AbstractIn estimating the vertical bearing capacity of spudcan foundation, the effect of the realistic lattice leg extending from the top of the spudcan is often ignored. This paper examines both t...

Bearing capacity of spudcan foundations in a spatially varying clayey seabed

Spudcan foundations are often pushed into a spatially varying non-homogeneous seabed to provide bearing capacity for a mobile jack-up platform. The natural variability of soil properties coupled with the complexity of loading conditions make determining the bearing capacity of spudcan foundations a challenging problem. A random finite element method is established to investigate the bearing capacity of a spudcan foundation embedded in a spatially varying clayey seabed when subjected to vertical, horizontal and moment loadings. A criterion is proposed for determining the characteristic value of the shear strength for the random seabed. Results indicate that the spatial variability in the clayey seabed significantly reduces the bearing capacity of a spudcan foundation. This reduction is more significant in the vertical bearing capacity than in the horizontal and moment bearing capacities. The mean bearing capacity is smaller for the clay with larger coefficient of variation of undrained shear strength. A characteristic value of mean minus a standard deviation of the undrained shear strength is capable to ensure the probability of failure is not greater than 5%. This study provide an evaluation method for the spatial variability effect of a clayey seabed, paving the way for a cost-effective design of spudcan foundations.

The bearing capacity of spudcan foundations under combined loading in spatially variable soils

Predicting the bearing capacity of a spudcan foundation under combined vertical (V), horizontal (H) and moment (M) loads is a challenging problem encountered by geotechnical engineers. In previous studies the combined VHM capacity was defined for a uniform soil profile, ignoring any variability in soil stratification and properties. In offshore conditions, however, both the soil profile and soil properties vary spatially. Therefore, it is of interest to account for the spatial variability of soil in the analysis of the bearing capacity of a spudcan. It is shown in this paper how the spatial variability of a clay affects the bearing capacity of a deeply buried spudcan foundation under combined loadings. Three-dimensional random fields are generated to model the spatial variability of undrained shear strength of clay and combined with a non-linear finite element analysis to investigate and define the VHM failure envelope of a spudcan foundation. Because of the random nature of soils VHM failure envelopes of different probability of occurrence are proposed. Results from this study provide guidance to the practical assessment of spudcan foundations in spatially varied soil conditions that can be encountered offshore.

Numerical investigation of bearing capacity due to spudcan penetration in sand overlying clay

Numerical studies were carried out to investigate the bearing capacity of spudcan foundations of offshore jack-up rigs in sand overlying normally consolidated clay by using the coupled Eulerian–Lag...

The effect of strength anisotropy on the bearing capacity of spudcan foundations

The vertical bearing capacity of spudcan foundations in strength anisotropic soils is investigated numerically using the MIT-S1 model implemented in the AFENA finite element package. The model in AFENA is validated against existing laboratory test data of normally consolidated soil. The bearing capacities of spudcans in soils with isotropic and anisotropic strengths are compared. Soil with isotropic strength is simulated using an elasto-plastic model. It is found that the bearing capacity of a spudcan in an anisotropic soil is reduced by about 9% for a rough spudcan and 3% for a smooth spudcan on average. There is a combined effect of soil anisotropy and spudcan roughness on the spudcan bearing capacity. Moreover, the effect of the pressuremeter strength of an anisotropic soil on foundation capacity can’t be ignored.